1. Field of Invention
The present invention relates to hydrophones employed in seismic exploration. More particularly, the invention relates to an improved hydrophone circuit that provides the electrical characteristics of a geophone by employing a second order high pass filter, wherein one capacitive element of the filter is a hydrophone.
2. Description of Related Art
Due to the increasing difficulty and cost of finding petroleum resources in the world today, exploration techniques are becoming more and more technologically sophisticated. For example, many have found crystal hydrophones to be useful in petroleum exploration. Basically, hydrophones are used to measure seismic waves created by a source such as an air gun or a dynamite charge, to obtain detailed information about various sub-surface strata of earth.
As shown in FIG. 1, a typical crystal hydrophone 100 includes a diaphragm 102, a crystal 104, and a housing 106 that is typically filled with a gas 107. The diaphragm 102, which has front and rear sides 102a, 102b, is made from a material such as Kovar.TM. or a Beryllium Copper compound, and is electrically connected to the crystal by a conductive epoxy 108. The crystal 104 is made from a material such as Lead Zirconium Titanate, and is silver-plated on its top 104a and bottom 104b to achieve better conductivity. The crystal 104 is initially polarized by applying a high-voltage electrical charge to the crystal 104. When the polarized crystal 104 experiences pressure resulting from a physical input such as sound, fluid pressure, or another type of pressure, it produces a voltage representative of the pressure experienced. The crystal 104 is electrically connected to electrical output leads 110, 112. To protect the crystal 104 from contaminants, and to maintain the crystal 104 in atmospheric pressure, the crystal 104 and the rear side 102b of the diaphragm 102 are sealed within the gas-filled housing 106. The housing 106 protects the crystal 104 and diaphragm 102, and facilitates mounting of the hydrophone 100.
The diaphragm 102 functions to vibrate in response to physical pressures it experiences. The physical deflection of the diaphragm 102 is transferred by the epoxy 108 to the crystal 104, deforming the electron structure of the crystal 104 and causing an electrical potential to be provided across the leads 110, 112.
Another apparatus that is also useful in petroleum exploration is the geophone. Geophones are commonly used to measure the motion of the earth's surface in response to seismic waves created by a seismic source, to obtain detailed information about various sub-surface strata in the earth.
As mentioned above, hydrophones and geophones are often used in petroleum exploration in conjunction with seismic equipment. In one example of such an application (FIG. 1B), a cable 150 including one or more hydrophones and one or more geophones is placed on the sea floor 154. Such a cable may be made up of cylindrical units 152, where each unit 152 includes a gimbaled geophone and a hydrophone. An example is the "ocean bottom cable" (OBC) system manufactured by Halliburton Geophysical Services; the OBC system utilizes six model SGH-2 hydrophone-geophone units, which are also manufactured by Halliburton Geophysical Services.
Seismic waves are produced by a seismic source 156 that is towed behind a ship; the seismic source 156 may comprise an air gun, a dynamite charge, or the like. The seismic source 156 produces a large explosion, creating seismic waves 160. The seismic waves 160 travel through water 162 and various layers of earth 164, and are reflected back to the cable 150 as upgoing incident waves 161. Each unit 152 detects and measures the incident waves 161 and creates a real-time record of the results. This record is typically stored in a recorder (not shown) that is linked to or contained within the cable 150. Records of this nature help geologists determine the makeup of the earth 164.
One problem with this arrangement, however, is surface ghost signals 166. Surface ghost signals 166 are produced by incident waves 161 that are reflected from the water's surface 168. At the wavelengths typically used for seismic signals, the surface 168 provides an effective mirror to reflect incident waves 161 and create downgoing surface ghost signals 166. Surface ghost signals 166 contain no additional information regarding the composition of the earth 164 or the possible petroleum deposits therein, and they interfere with the proper receipt and interpretation of the incident waves 161. Accordingly, it is desirable to eliminate the errors introduced by the surface ghost signals 166.
A hydrophone-geophone combination, in theory, is naturally suited to eliminate surface ghost signals. Generally, hydrophones detect pressure omnidirectionally, and geophones detect velocity, which is directional. Due to the relative strengths of the incident waves 161 and the surface ghost signals 166 at different depths, a hydrophone's output and a geophone's output will both vary with depth. For a seismic wave 161 of a given magnitude and frequency, a hydrophone's output will vary with depth sinusoidally (curve 180, FIG. 1C). Likewise, for the given seismic wave 161, a geophone's output will vary sinusoidally with depth (curve 182, FIG. 1C). The hydrophone and geophone outputs may be scaled by external circuitry or by a mathematical algorithm in a computer, so that their peak values have the same amplitude; for example, in FIG. 1C, the hydrophone and geophone outputs are scaled to a maximum peak amplitude of 1 and a minimum peak amplitude of -1. After such scaling, the sum of the hydrophone and geophone outputs will always be 1, irrespective of the depth at which the hydrophone and geophone are both located (curve 184, FIG. 1C). Therefore, in theory, a hydrophone output and a geophone output may be combined to effectively eliminate the influence of surface ghost signals 166.
One problem in applying this theory is that the frequency responses of hydrophones and geophones differ. Therefore, the hydrophone and geophone outputs will only complement each other as shown in FIG. 1C when the seismic wave 160 has a certain frequency. As a result, if the frequency of the seismic wave 160 were to change, the combined hydrophone-geophone output 184 would no longer be constant.
The difference between frequency responses of hydrophones and geophones will now be explained with reference to FIGS. 2-5. When an electronic amplifier 200 (FIG. 2) is utilized to amplify the output of a typical hydrophone 202, the frequency response of the hydrophone 202 (FIGS. 3A, 3B) resembles that of a single-pole high pass filter, since it exhibits a single pole and a 6 dB/octave slope at frequencies less than its natural frequency (f.sub.n). The amplifier 200 may comprise an operational amplifier. The hydrophone may be modeled as a voltage source 202a and a capacitor 202b in series; the capacitor 202b and resistance 204 provide the single pole, and hence the 6 dB/octave slope. The natural frequency of the hydrophone 202 depends upon the value of the internal resistance 204 (R) of the amplifier 200 and the capacitance (C) of the capacitor 202b; this relationship is shown in equation 1.0, below. ##EQU1##
In contrast to the hydrophone 202, the frequency response of a typical geophone resembles a double-pole high pass filter with a 12 dB/octave slope at frequencies less than its natural frequency (FIGS. 4A, 4B). The geophone's frequency response generally explained can be understood by considering the structure of a geophone. A typical geophone (not shown) includes a coil of wire that is positioned within a magnetic field and contained within a case. The coil is suspended by springs and tends to remain fixed in space due to its mass, while the case and magnetic structure vibrate with the motion of the earth's surface. The relative movement of the electrical coil within the magnetic field induces an electrical voltage across the coil, representative of the motion of the earth's surface. A damping resistor is connected across the coil to create a loop and thereby permit current to flow through the coil in proportion to the coil's velocity relative to the magnetic field. This flowing current has the effect of damping the coil's motion due to the force placed on the electrified loop by the magnetic field. This damping facilitates a more accurate representation of the seismic signals received by the hydrophone.
This mechanical system may be modeled as an electromechanical system, a typical application of which is shown in FIG. 4C. A geophone model 400 is electrically connected to a damping resistor 402, as discussed above, and an amplifier 404. The geophone model 400 includes a voltage source 406 to represent the velocity of the geophone case multipled by the transduction constant of the geophone. The model 400 also includes a capacitor 408 to represent the mass of the moving coil (i.e., C=aM). An inductor 410 is shown to represent the spring constant of the coil suspension (i.e., L=a/k). The model 400 also includes a first resistor 412 to represent the "coil form damping", i.e., the mechanical damping of the moving coil caused by eddy currents therein. A second resistor 414 represents the electrical resistance of the coil. The damping resistor 402, in combination with the input resistance of amplifier 404 and the coil resistance 414, is selected to electrically damp the geophone's response, as discussed above. In an exemplary embodiment, the damping resistor 402 is chosen to achieve 70% of critical damping. The geophone's natural frequency is calculated as shown in equation 2.0, below. ##EQU2## A geophone's frequency response, then, resembles a high pass second order electronic filter, since the geophone's equivalent circuit contains two impedance elements: the capacitor 408 and the inductor 410. Although the geophone model 400 acts like a resonant circuit, the output signal at the natural frequency is typically 3 dB lower than the asymptotic value due to the 70% damping provided by the damping resistor 402 in combination with the input resistance of the amplifier 404 and the coil resistance 414.
For the reasons explained above, hydrophones and geophones have different frequency response characteristics. Accordingly, hydrophones and geophones are not naturally suited to eliminate ghost signals 166 across the whole spectrum of desired frequency. To use a hydrophone with a geophone advantageously, the frequency response of the hydrophone must match the frequency response of the geophone. One approach to this problem has been to cascade the output of the op-amp 200 with a circuit 500 (FIG. 5) that includes a resistor 502 and a capacitor 504. However, the frequency response of such a circuit still differs from the frequency response of a geophone. Even though the addition of the capacitor 504 provides a second pole, the modified hydrophone circuit is still not a resonant circuit, and therefore the gain at the hydrophone's natural frequency is 6 dB down rather than 3 dB down.
Due to the limitations of this approach, some have used transformers in conjunction with hydrophones to obtain a frequency response that matches that of a geophone. Although a transformer may be advantageously used to bring the output of the hydrophone into conformance with that of a geophone at a particular output level of the hydrophone, there are a number of significant limitations. In particular, the magnitude and phase of the signal produced by the transformer-coupled hydrophone varies with the magnitude of the electrical output signal produced by the hydrophone, due to non-linearities of the transformer core. Furthermore, the natural frequency of the transformer-coupled hydrophone also varies with the magnitude of the electrical output signal provided by the hydrophone. FIG. 6 shows how the natural frequencies of two sample transformer-coupled hydrophones vary over a range of different hydrophone output amplitudes. The tests of FIG. 6 were conducted with an Oyo MP-24 transformer-coupled hydrophone. The use of a transformer to configure the output of a hydrophone to that of a geophone, then, is limited, since the hydrophone's output will not compliment the geophone's output for all amplitudes of the hydrophone's operating range.
This is more clearly illustrated in FIGS. 7 and 8, which show the frequency response of a conventional transformer-coupled hydrophone. In FIGS. 7 and 8, one sample curve was taken with a first seismic input, and the other sample curve was taken with a second seismic input ten times greater than the first.
In FIG. 7, the horizontal axis represents frequency in increments of 1 Hz. The vertical axis represents the magnitude of the transformer-coupled hydrophone's output. In particular, the voltage output of the transformer-coupled hydrophone was measured and compared to the voltage output of an arbitrarily selected, standard hydrophone with excellent low frequency response. In other words, the voltage output of the standard hydrophone was constant over the range of frequencies tested. The outputs of the standard hydrophone and the transformer-coupled hydrophone under test were compared using the formula of equation 3.0, below. ##EQU3## As seen in FIG. 7, the relative magnitude of the transformer-coupled hydrophone varied when the seismic input signal was increased.
In FIG. 8, the horizontal axis represents frequency in increments of 1 Hz. The vertical axis represents the phase of the transformer-coupled hydrophone's output, in degrees. FIG. 8 illustrates that the phase of the transformer-coupled hydrophone varied when the seismic input signal was increased.
In addition to the transformer-related problems mentioned above, transformers are not always as cost efficient as might be desired. Moreover, transformers are not always as free from distortion as some might require; transformers typically suffer from non-linearity due to power losses caused by hysteresis, eddy currents, "I.sup.2 R", and other inefficient characteristics.